Alternating current power bridge



May 7, 1946. Av E. HARRISON 2,399,674

ALTERNATING CURRENT POWER BRIDGE Filed Nov. 11, 1943 INVENTOR ARTHUR E.HARP/SON ATTO RN EY Patented May57, 1946 UNITED STATES PATENT orrlcaALTERNATING CURRENT POWER Arthur E. Harrison, Roekville Centre, N. Y.,as-

aignor to Sperry Gyroscope Company, Inc., a corporation or New YorkApplication November 11, ms, Serial No. 509,889

11 Claims.

The present invention relates to power measurement and has for itsprincipal object the provision of improved methods and apparatus formeasuring power, particularly at high frequencies.

An object of the invention is to,provide a balanced bridge wattmeterrequiring only one defleeting instrument and having a relatively smalldirect-current power input, consuming only about one-fourth the directcurrent power usually consumed in an alternating-current power bridge.

Another object is'l, to provide a power bridge in which simplifiedrectifier and filter arrange-v ments may be employed or which-may beenergized by a storage battery-or dry cells.

Still another. object is 'to5provide a bridge-inwhich no galvanometer isrequired-'"asfa null indicator.

Still another object is to priivide bridge having a hot wire element,the'resis'tance oi. vi hich is maintained constant to maintain."v'olta8e and amperage proportional. V An object of the invention is toprovide a direct reading radio frequency wattmeter which is sensitive tosmall changes of large powers and which has a calibration curverepresented by the formula l-x. i

Another object is to providea bridge which is adjustable for employinghot wire elements of different resistances.

Still another object of the invention is to provide a bridge employingonly one deflecting instrument and in which dipping of the instrumentpointer is avoided.

Still another object of the invention is to provide a power bridgehaving a constant standing I wave ratio regardless of the magnitude ofthe power, but which has a relatively simple circuit.

Still another object of the invention is to pro-' vide a power bridge inwhich direct-current power consumption is reduced by making resistancesof the bridge unsymmetrical and dissipating most of the power in one armof the bridge.

Still another object of-the present invention is to provide a powerbridge in which balance my be ascertained by closing or opening thediagonal arm without utilizing a null galvanometer.

Other and further objects and advantages will become apparent as thedescription proceeds.

In carrying out the invention in its preferred form, a hot wire elementis employed which carries alternating current, the power or which is tobe measured. Means are provided for supplying the hot wire with directcurrent power, for ascertaining the direct current power consumptionwhen alternating current power is applied and when alternating currentpower is absent, and formaintaining the total power dissipation constantin order that the diflerence in the two values of direct current powerwill represent the alternating current power measured. In order tomaintainthe power. dissipation constant, means are provided formeasuring the resistance oi! the hotwire and holding it at a constantvalue. To this end, the hot wire ismade one arm of a resistance bridge.

In order to maintain the power consumption 1 of the bridge at a minimum,one or more arms oi.

the bridge aremade very small in resistance, and

Us. miiliammeter serving as a power indicating instrument is made onearm of thebridge. In .order to eliminate the need for a null indicationgalvanometer, a switch is connected with thedl- .agonal arm of thebridge and is arranged to be i opened or closed to indicate presence orabsence of bridge balance by absence or presence of variation in thereading of the miiliammeter serving as one arm or the bridge.

A better understanding ofthe invention will be afiorded by the iollowingdetailed description, considered in connection with the accompanyingdrawing, in which a Fig. 1 is a circuit d ram of a balancedpowermeasuring bridge Fig. 2 is a circuit diagram of the bridge with amilliammeter shown as one arm or the bridge and Like referencecharacters areutilized througha hot wire alternating currentpower-absorbing element schematically represented as another arm of thebridge:

Fig. 3 is a more detailed circuit diagram showing an arrangement foralternatively calibrating the bridge or connecting it for measurement ofpower; and

Fig. 4 is a diagram illustrating the scale dis tribution which may beobtained in deflecting the current instrument serving as a powerindicato in the apparatus of Figs. 2 and 8.

out the drawing to designate like parts.

Balancedbridges have been proposed for measuring alternating currentpower in which one arm 01' the bridge receives both direct current powerfrom a source energizing the bridge and alternating current power fromthe source to be i measured. Any variations in alternating current powerare ascertained by measuring the varla-i tions in direct current powerrequired to balance the bridge.

Ordinarily, the four bridge arms are made equal in resistance so as' toproduce what is called a "symmetrical'bridge." With equal resistancesthe same power is consumed in each bridge arm, and the total directpower consumed must be four times that required to be consumed in thehot 'wire making the bridge unsymmetrical, thereby reducing theunnecessarily'great sensitivity of the balance indication.

For example, referring to Fig. 1, the bridge may comprise three unequalbridge arms R1, Ra, R: and a fourth bridge arm Rw representing thedirect current resistance of a hot wire adapted to receive highfrequency alternating current power. For energizing the bridge withdirect current, a suitable source I l, schematically represented as abattery of dry cells, is provided. As shown in Fig. 1, the conventionaldiagonal arm consists of a deflecting instrument which may be amilliammeter 12 having a resistance Rm connected between two conjugatepoints it and it of the bridge. The remaining two conjugate points itand it of the bridge are connected to the direct current power source II.

For the sake of conserving direct current power, the resistance ratioRl/Rw is made large and the ratio Rw/Ra is also made large. Suillcientnull sensitivity is obtained if the resistance of the arm R: is verylarge in comparison with the resistance Rm of the instrument l2.

For avoiding the necessity of separate instruments for indicatingbalance and measuring direct current input, the instrument I? may beconnested in place of the bridge arm Re. This is iiiustrated in Fig. 2.For indicating bridge balance, a switch I! is connected between theconjugate points It and ll of the bridge. As shown, the hot wire arm Rwof the bridge is so mounted as to absorb radio frequency power suppliedthereto through a concentric line unit ll.

The unit l8 comprises an outer metallic cy1ln-' der is, an innerconductor 2|, a continuation of which is formed by the hot wire Ray anda quarter-wave stub 22, permitting a direct current circuit to be madethrough the wire R' without upsetting the standing wave relationships inthe concentric line unit It. A suitable coupling (not shown) is providedfor supplying microwave alternating current power at the right-hand end23 of the concentric line I! (shown broken away). For short-circuitingthe left-hand end 24 of the line It with respect to alternating currentbut insulating the inner and outer conductors with regard to directcurrent, a cylindrical condenser is formed at the left-handend 24,comprising a conducting cylinder 25 surrounded by an insulating tube 28,which is surrounded in turn by a metallic sleeve 21 making contact withthe inner surface of the hollow cylinder ll. Direct current connectionsare made through conductors 28 and 2! to the conducting-elements :I and21, respectively.

In Fig. 2 as in Fig. 1, the resistance ratios Ri/Rw and Rw/Rs are large,of the order of 100:1, for example. Furthermore, the resistance Rm,though small, is large in relation to the negligible resistances betweenthe points I! and it when switch I I is closed.

When the bridge of Fig. 2 is balanced, no current tends to flow throughthe diagonal arm, and the reading of the miiliammeter i2 is unaflectedby opening or closing the switch II. A balanced condition of the bridgeis therefore indicated by absence of variation in instrument readingwhen the switch I1 is opened and closed.-

The balance indication sensitivity is equivalent to the accuracy of .thesame instrument. In addition, there is no uncertainty in regard to thenull, and parallax would not aflect' the condition of balance, sincebalance is indicated by no deviation from the previous position of theinstrument pointer. I

For adjusting the magnitude of the direct current,input to the bridgefor the purpose of balthe resistances being maintained constant, theinstrument l2 may be calibrated in terms of power. Preferably, theapparatus is so arranged that a maximum current reading or minimum powerreading, as shown by the scale represented in Fig. 4, is obtained whendirect current power only is suppliedto the hot wire element Rw.

Since the resistance of the hot wire element Rw depends upon itstemperature, which in turn depends upon the power consumed in it, themaintenance of constant resistance by maintaining bridge balance assuresthe constancy of the power consumption in the elementRw whether thepower is supplied partially or wholly by direct current.

The bridge may be either by the short-circuit method or the customarydiagonal galvanometer method, and, furthermore, calibrating means may beprovided for enabling the bridge to be used with hot wire units ofdifferent resistances as illustrated in Fig. 3.

In the arrangement of Fig. 3 a change-over switch 32 is provided, havingan upper position for utilizing the instrument l2 to measure power and alower position for utilizing the instrument M to indicate the nullcondition.

Furthermore, a pair of 34 is .provided for alternatively connecting theapparatus for calibration, which is the left-hand gosition, and use,which is the right-hand posi- To facilitate calibration, a resistance R6is pro vided which is adapted to be connected inseto obtain balance whenno arranged for being balanced anged switches 33 and ascacva mainingends of the resistors Rs and R1 being connected to the bridge arm R1.Likewise, the calibration switch 34 comprises a movable blade .35connected to the bridge point l4 and a pair of stationary contacts 39and 4| connected tothe ends of the resistors Re and Rs, respectively,

the remaining ends of which are connected to the bridge point It commonto the bridge arms R: and Re.

A suitable mechanical connection represented by the dotted line 42 isprovided for causingsimultaneous operation of the calibrator switchblades 33 and 34.

The change-over switch 32 comprises a singlepole, double throw element43 and a single-pole single throw element 44 ganged together asrepresented by dotted line 32'. The element 43 has a movable blade 45connected to the bridge point H at right-hand end of the'resistor R1, anupper stationary, contact 45 connected to a left-hand instrumentterminal 41, and a lower stationary contact 48 connected to a terminal49 common to the instrument i2 and the resistor R2.

The element 44 has a stationary contact 50 connected to the terminal 49and a movable blade 5| connected to the bridge point It.

When the calibration switches 33 and 34 are moved to the left-hand orcalibration position,

circuits are closed from the bridge point 15 through the switch elements35 and 38, a conductor 52 and the resistor Rs to the bridge arm R1. andlikewise from the bridge point vl4, through the switch elements 35 and35, and the resistor Re to the bridge point l5, placing the resistor Rin shunt with the resistor Ra. On the other hand, when the calibrationswitches 33 and 34 are moved to the right or use position, circuits arecompleted from the bridge point 15 through the switch elements 35 and31. a conductor 53, and the rheostat 51 to the bridge arm Ra, thusplacing the rheostat R': instead of the resistor Rs in series with theresistor R1. Likewisaa circuit is completed from the bridge point l4,through the switch elements 38 and 4|, through the rheostat R: to thebridge point l5, placing the rheostat Ra instead of the resistor Rs inparallel with the bridgearmRa.

when the change-over switch 32 is in the lower or null indicatingposition, the bridge arm R: is connected between the bridge points I andI3 and in series with the resistor R1 through the terminal 49, aconductor 54, conductor 55, switch elements 48 and 45, and a conductor58 to the bridge point l3 and the resistor Ri. Likewise, a diagonal armcircuit is completed from the terminal through the instrument 12, aconductor and the switch H to the conjugate point i4. 0n the other hand,when the change-over switch 32 is moved to the normal upper or powermeasuring position, the contacts 50 and 5| are closed, short-circuitingthe resistor R: and substituting the instrument l2 for the resistor R:in a cirf cuit from the bridge point IS, the switch contacts 50 and5|..the conductor 54, the instrument j R. F. power reading will increaseas the R. F.

Several power ranges may be combined in a single unit. with a selectorswitch connected to diflerent resistors in the bridge, but for the sakeof simplicity, the multi-brldge feature has been deleted in the circuitof Fig. 3. The operating procedure and the calibrating adjustments areas follows:

(1) With switch 3334 in the calibrate" position, increase the D. C.input by means of the rheostat 3| until the rated value is reached. Thisis the value at which the instrument I2 reads 0 on the scale of Fig. 4.Resistors R5 and Re are chosen so that the combination with R1 and R3will balance the bridge if the hot wire resistance Rw is the rated valueat the rated power input. The meter should read full scale (zero power)with switch l1 open or closed. If the hot wire resistance differsslightly from th rated value at the rated power input, the bridge willnot be balanced. In this case, adjust the D. C. power input until thereadings with switch I! open and closed are equally spaced from the fullscale (zero power) position. The power input for this adjustment will berated power if the deviation from full scale reading is not too great.

(2) Without changing the power input, return switch 3334 to the use"position, close the switch l1 and adjust Ra until the meter reads fullscale (zero power). Open switch I! and adjust R1 until the meter againreads full scale. The bridge is now balancedand the power reading iszero.

(3) Apply the R. F. power. It the R. F. power is known to be large,reduce the D. C. input first to avoid danger of burning out the hotwire. Maximize the power input. This can be determined by observing theefiect on the meter reading. The D. C. current will decrease, i. e., the

power increases the resistance of the hot wire (4) Openswitch IIintermittently while adjusting the D. C. power input until the meterpointer is motionless when the switch is opened. Make any finaladjustments of R. F. matching, etc.,g repeating the balance adjustmentif necessary. The R. F. power is then read directly on the meter scale.

(5) If a null indication is desired, switch 32 should bechanged from thepower position to the null position. After the R. F. adjustments havebeen made, and the D. C. power reduced to give'a null indication, switch32 is returned to the power position and the meter scale will read thevalue of the R. F. power.

(6) Any failure to obtain a bridge balance at retaining thedirect-reading feature and having the bridge balanced even thoughhot-wire ele- I2, the conductors 58 and 55, the switch contacts 45 and45, and the conductor 58 to the bridge point I! and the resistor R1. Thediagonal arm of the bridge is then the switch ll between the points l3and I4.

It will be understood that the balance-testin switch I1 is closed whenbalance is to be tested by means of the instrument l2, and the switch 32is in the null position. 1

ments of precisely the same resistance are'not always available.

In accordance with the provisions of the patent statutes, the principleof the operation of the invention has been described together with theapparatus now believed to represent the best embodiment thereof, but itis to be understood that the apparatus shown and described is onlyillustrative and that the invention may be carried out by otherarrangements.

What is claimed is; 1. A low-powerconsumption bridge, which comprisesfirst, second and third bridge resistors. a hot wire element connectedin series-parallel with saidresistors to form a Wheatstone bridgecircuit, and a galvanometer. he first and second resistors beingconnected in series, and the hot wire element and the third resistorbeing connected in series, the first resistor and the hot wire elementhaving a common terminal, and the second and third resistors having acommon terminal, the galvanometer being connected as a diagonal armacross said second and third resistors, the resistance of the firstresistor being high in relation to the hot wire element, and theresistance of the hot wire element being high in relation to theresistance 01' the third resistor, the resistance ratio of the first andsecond resistors being equal to the resistance ratio of the hot wireelement and the third resistor to give bridge balance, and theresistance of the second resistor being high in relation to theresistance of the diagonal arm galvanometer.

2. A power measuring bridge for alternating current, comprising a hotwire element, a first resistor, a current-responsive instrument havingan internal resistance serving as a second resistor, a third resistor,and a switch, the three resistors and the hot wire element beingconnected in series-parallel to form a bridge, the first resistorbration resistors in the calibration-position oi the switch andconnecting the operating rh'eostat in the use position or the switch.

5. A power measuring bridge for alternating current, comprising a hotwire element, a first resistor, a second resistor, a current-responsiveinstrument having an internal resistance serving as a resistor, and athird resistor, the three resistors and the hot wire element beingadapted to be connected in series-parallel to form a bridge, but thefirst and third resistor, the hot wire element and the instrumentnormally being in seand the instrument having a common terminal servingas one conjugate bridge point, the hot wire element and the thirdresistor having a com-\ mon terminal serving as a second conjugatebridge point, the switch being connectedbetween said conjugate bridgepoints for indicating balance of the bridge by an absence of variationin deflection of the instrument, the resistance of the first resistorbeing high in relation to the resistance of the hot wire element, theresistance of the hot wire element being high in relation to theresistance of the third resistor, and the resistance of the instrumentbeing relatively high in relation to the resistance between said pointswhen said switch-is closed.

3. A power measuring bridge for alternating,

current, comprising a hot wire element, a first resistor, acurrent-responsive instrument havingan internal resistance serving as asecond resistor,

switch, the three resistors a third resistor, and a conjugate bridgeries-parall'el, the first resistor and the instrument, having a commonterminal serving as one conjugate bridge point, the hot wire element andthe third resistor having a common terminal serving as a secondconjugate bridge point, a test switch being connected between saidconjugate bridge points for indicating balance of the bridge by anabsence of variation in deflection oi' the instrument, a change-overswitch, said change-over switch having contacts for alternativelyconnecting the second resistor in place of the instrument and connectingthe instrument in series with said test switch between the conjugatebridge points for null indications.

6. An alternating current power measuring bridge comprising tourresistance elements connected in series-parallel, one of said elementsbeing a hot wire element adaptedto absorb alternating current ower, oneof said elements being a deflecting instrument, and the resistance ratioand the hot wire element being connected in se-- ries-parallel to form abridge, the first resistor and. the instrument having a common terminalserving as one conjugate bridge point, the hot wire element and thethird resistor having a common terminal serving as a second conjugatebridge point, the switch being connected between said conjugate bridgepoints for indicating balance of the bridge by an absence .of variationin thedeflection of the instrument.

4. A power measuring bridge for alternating current, comprising a hotwire element, a. first resistor, a second resistor-means, and a third.re-

sistor, the three resistors and the hot wire element being connected inseries-paralle1 to form a bridge, the first resistor and the secondresistor means having a common terminal serving as one conjugate bridgepoint, the hot wire element and the third resistor having calibrationand use positions with contacts for connecting said'calia commonterminal serving as a second conjugate bridge point, a callparallel withsaid third re-- ner such that a major being high.

7. A resistance-responsive bridge circuit, comprising four resistanceelements connected in series-parallel, each series pair having a commonterminal serving as a conjugate bridge terminal, and a switch connectedbetween said bridge terminals, one of said bridge arms including acurrent-responsive instrument whereby balance is indicated on openingand closing said switch by constancy of instrument indication.

8. An alternating-current power measuring bridge, comprising a pluralityof resistance elements connected in series-parallel, with one of saidelements being a hot wire element adapted to absorb alternating currentpower, the resistance ratios oi said elements being such as to cause apreponderant proportion of the energy to be absorbed in the hot wireelement.

9. An alternating current power measuring bridge comprising a hot wireelement and three resistance elements connected in series-parallel toform a bridge circuit, the resistance of one element adjacent said hotwire element being greater than the resistance oi said hot wire elementand the resistance 01' the other element adjacent said hot wire elementbeing smaller than the resistance 01' said hot wire element in amanportion of the energy supplied to said bridge circuit is dissipatedby said hot wire element.

10.. An alternating current power measuring bridge, comprising a hotwire element and three resistance elements connected in series-parallelto form a bridge circuit having two conjugate terminals for connectionto a potential source, the resistance of the one of said resistanceelements conneted to said hot wire element at one 01' said conjugateterminals being greater than the resistance or said hot wire element,and the resistance oi the other element connected to said hot wireelement being smaller than the resistonce or hot wire element, whereby amaior to term a. bridge circuit. the resistance of one portion of theenergy supplied to said brldze cirelement adjacent said hot wire elementbeing emit icy MM; potential source is dissipated by said greater thanthe resistance or said hot wire eiehot n 1 element. ment and theresistance oi the other element adn. m alternating current powermeesurinz- 5 jacent said hot wire element being smaller than mimeccmpriainz a. hot wire element and three the resistance or said hot wireelement. reeintcc elements-connected in series-parallel ARTHUR E. SON.

